1. Field of the Invention
The present invention relates to a local area network (LAN), more specifically to the optimization of communication quality for realizing an optimal communication environment between a wireless LAN satellite station and a master station in a wireless LAN system mainly using an electromagnetic wave in a millimeter wave range.
2. Description of the Related Art
These days a LAN has become popular and the amount of data handled on a LAN has been also increasing. On the other hand, data terminals including a personal computer have been improved in communicating functions, and have been miniaturized. Under these circumstances, the need for data communication in a mobile environment is advocated, and attention is being paid to a wireless LAN system.
So far an electro-magnetic wave in an ISM range, that is, approximately the 1 to 3 GHz range, has been used for a wireless LAN system. However, since this bandwidth is also used for industrial purposes or electronic ovens, a large amount of noise is generated. To suppress this large amount of noise it is necessary to employ a spread spectrum communication method, which has made a system complicated. The system also has a drawback that in this wave range a bandwidth required for a high speed transmission cannot be secured.
For this reason, attention is currently being paid to a millimeter wave range of 50 to 70 GHz which is still an unused wave range for data communication.
An electro-magnetic wave in this wave range is characterized in that it is strong to keep straight on, and it is easily absorbed by oxygen and glass. For example, since there is little possibility that an electro-magnetic wave leaks outside when it is used in an office environment, it is effective in security. Furthermore, since it is an unused band width, and thereby there is no need to employ a spread spectrum communication method, the system can be simplified, compared with the case when an ISM range is used. Since in a millimeter wave range a band with required for a high speed transmission can be secured, it is a very promising wave range at present when a high speed transmission of over 100 Mbps is becoming popular in a wired LAN system.
However, the wireless LAN system using an electro-magnetic wave in a millimeter range has the following problems.
The first problem is that a communication environment rapidly deteriorates due to a subtle change of the position and direction of an antenna of a satellite station. In order to expand a communication-available area in a millimeter wave wireless LAN system an antenna with a rather broad radiating directivity is usually used for the master station. However, a millimeter wave is strong to keep straight on and is easily absorbed by oxygen. There is also influence from interference among satellite stations and multipath interference which is caused by being a plurality of routes for an electro-magnetic wave transmitted from a satellite station. For this reason, to secure a favorable communication environment, it is necessary to sharpen the radiation characteristic of the antennae of satellite stations and to always direct the radiation characteristic of the satellite stations exactly to the antenna of the master station.
To avoid the influence from interference among satellite stations and multipath, the use of diversity antennae and the introduction of a spread spectrum communication method can be considered. However, it is technically difficult to implement these methods in a millimeter wave range, and even if it can be implemented, the system becomes complicated. This is the second problem.
It is an object of the present invention to provide a millimeter wave wireless LAN system in which a favorable communication environment can be secured between a satellite station and a master station by directing the radiation characteristic of an antenna of a wireless LAN satellite station exactly to an antenna of the master station in view of the above-mentioned problems.
To attain the object of the present invention as described above, a wireless LAN system of the present invention comprises a wireless LAN master station for supporting communication between satellite stations belonging to the master station and one or more wireless LAN satellite stations. A transmitter-receiver in the system being a satellite station of the LAN system comprises an antenna, the directivity characteristic of which can be dynamically changed when receiving waves from the master station, control frame transmitting means for transmitting a control frame to the master station prior to the commencement of communication, and antenna directivity characteristic controlling means for determining such a directivity characteristic that the receiving electric field intensity of a carrier wave transmitted from the master station when the control frame is transmitted may become a maximum by changing the directivity characteristic of an antenna. A transmitter-receiver for communicating with another party in the system being the master station comprises carrier wave transmitting means for transmitting a carrier wave when receiving the control frame. In the wireless LAN system of the present invention, prior to the commencement of communication, a satellite station transmits a control frame to the master station. When the master station receives the control frame from the satellite station, it starts to transmit a carrier wave. When the satellite station receives the carrier wave transmitted from the master station, it changes the directivity characteristic of an antenna, and determines such a directivity characteristic that the receiving electric field intensity of the carrier wave may become a maximum. After that, by using the directivity characteristic obtained from the result of the determination, communication between a satellite station and the master station can be carried out in an optimal communication environment.
For an antenna of a satellite station an active phased planar-array antenna can be used. Thus, without adjusting the physical position of an antenna of a satellite station, communication between a satellite station and the master station can be carried out in an optimal communication environment.
Further, after the satellite station starts to exchange data frames as communications with the master station, conditions can be set for the number of errors detected in a data frame that are allowed to be received, or the electric field intensity of the data frame that is allowed to be received, and when the conditions are not met, an optimal antenna directivity characteristic can be determined again. Thus, the deterioration of communication quality occurring because a satellite station or the master station moves or because the position of an antenna is shifted due to some cause, can be automatically coped with, and an optimal communication environment can be always provided.
Furthermore, the transmitting power of a carrier wave which is transmitted when the master station receives a control frame from a satellite station, can be less than the transmitting power at the time of the transmission of a data frame as normal communication. Thus, the power consumption of the master station can be reduced.